The invention of U.S. patent application Ser. No. 09/932,703 discloses a system and method for interconnecting processing units of a stored program controlled system using free space optics. According to this disclosure, an optical beam line carries signals among the various processing units. Multiple distinct streams of data are communicated within that system. When multiple distinct communication streams within a system or network share the same physical media, a multiplexing and Media Access Control (MAC) protocol is necessary for optimal system operation. These protocols help insure that the system performs at the desired capacity, performance and reliability levels.
One important function of any multiplexing and MAC protocol is to divide the limited capacity of the shared physical communication channel among the various logical streams or subchannels that share it. This subdivision of the shared physical channel provides capacity guarantees, so that each of the logical channels receives an equal portion of the available capacity, or a pre-arranged larger or smaller proportion of this capacity. Pre-allocation of capacity provides a guaranteed level of capacity to all users. Such pre-allocation is often wasteful of system capacity because, if a given subchannel has no traffic to send, other subchannels may have more traffic than they can handle.
Other multiplexing and MAC protocols provide statistical multiplexing of the system's capacity, wherein all potential transmitters on the shared channel negotiate for an opportunity to transmit. The theoretical maximum load offered, if all transmitters are operating at full rate, often exceeded the capacity of the shared media, which requires various buffering, queuing and priority techniques to govern when each transmitter operates. This system has potential channel utilization efficiency advantages, at the expense of making it more difficult to guarantee a minimum individual subchannel capacity or latency.
Another important function of any multiplexing and MAC protocol is to direct or route a given channel's traffic to the intended receiver over the shared medium. Channels in an optical system are often statically mapped so that each subchannel operates in its own space (or similar separators as known in the art), and the receiver is determined by the location in space assigned to the transmitter. In other cases, traffic is offered to the shared medium as packets. These packets contain a destination address, which is used by switching or routing functions to complete the connections.
The concept of priority is also important to multiplexing and MAC protocols. Some messages have a higher priority than others, based upon the importance or time sensitivity of their contents. The multiplexing and MAC protocol must take priority into account as it manages access to the shared medium.
Security is often an intrinsic function of multiplexing and MAC protocols. If there is a hazard associated with unauthorized interception of messages over the shared medium, the protocol can take steps to eliminate (or at least greatly reduce the impact of) this hazard. One means for heightened security is physically separating sensitive traffic from all other traffic in the various parallel submedia in the transmission medium. A more common approach is to use some form of encryption at the transmitter, and decryption at only the authorized receiver(s).
A further function that multiplexing and MAC protocols provide is fault tolerance and fault recovery. If a failure occurs that disables the shared media or significantly reduces its capacity, the protocol invokes various diagnostic actions to discover the source of the problem and then appropriate recovery actions to attempt to correct the source of the problem. Often, the fault recovery operation involves switching the traffic to a redundant medium or attempting to shed load so only the highest priority traffic is allowed on the remaining capacity.
One form of multiplexing and MAC protocols usable in a free space optical beam line medium is space division multiplexing. In a Space Division Multiplexed (SDM) system, the shared medium is divided into a set of simultaneously transmitted beams, all occupying different subvolumes of the shared beamline volume. Each subchannel sharing the medium is assigned a specific beam with a specific spatial position on which it carries traffic. The number of beams assigned per subchannel may be equal for all subchannels, or some subchannels may be given higher capacity by assigning them a higher proportion of beam volumes on which to transmit.